Nonaqueous electrolytic solution and lithium secondary battery

ABSTRACT

A non-aqueous electrolytic solution containing a ketone compound having the formula (I):  
                 
 
     [each of R 1  and R 2  independently represents a linear or branched alkyl group; and each of R 3 , R 4 , R 5  and R 6  independently represents a hydrogen atom or a linear or branched alkyl group; provided that R 1  and R 4  can be combined to form a cycloalkanone ring in conjunction with a propanone skeleton to which R 1  and R 4  are connected, that two or more of an alkyl group of R 2 , an alkyl group of R 5 , a branched chain of an alkyl group of R 1 , and a branched chain of an alkyl group of R 1  can be combined to form a cycloalkane ring, or that an alkyl group of R 1  and an alkyl group of R 2  and/or an alkyl group of R 1  and an alkyl group of R 5  can be combined to each other to form a cycloalkane ring] 
     is favorably employed as a constituent material for preparing a lithium secondary battery that is excellent in the battery performances and cycle performance.

FIELD OF THE INVENTION

[0001] The present invention relates to a lithium secondary batteryshowing excellent performances in safety under the overchargeconditions, cycle performance, electric capacity and storage endurance,and further to a non-aqueous electrolytic solution favorably employablefor preparing the lithium secondary battery.

BACKGROUND OF THE INVENTION

[0002] Recently, a lithium secondary battery is generally employed as anelectric source for driving small electronic devices. The lithiumsecondary battery mainly comprises a positive electrode, a non-aqueouselectrolytic solution, and a negative electrode. The non-aqueous lithiumsecondary battery preferably employs a positive electrode of lithiumcomplex oxide such as LiCoO₂ and a negative electrode of carbonaceousmaterial or lithium metal. The non-aqueous electrolytic solution for thelithium secondary battery preferably employs carbonate compounds such asethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate(DMC), diethyl carbonate (DEC), and methyl ethyl carbonate (MEC).

[0003] Nevertheless, it is desired to provide a secondary battery havingmore excellent performances in the cycle performance and electriccapacity.

[0004] In the lithium secondary battery using a positive electrode ofLiCoO₂, LiMn₂O₄, or LiNiO₂, oxidative decomposition of a portion of asolvent of the non-aqueous electrolytic solution takes place in theelectric charging stage. The decomposition product disturbselectrochemical reaction of the battery so that the battery performancelowers. It is considered that the oxidative decomposition occurs in thesolvent on the interface between the positive electrode and thenon-aqueous electrolytic solution. Therefore, the battery performancessuch as cycle performance and electric capacity are not satisfactory forbatteries to be repeatedly charged and discharged at a maximum workingvoltage exceeding 4.1 V.

[0005] Moreover, in the lithium secondary battery using particularly anegative electrode of carbonaceous material of high crystallinity suchas natural graphite or artificial graphite, reductive decomposition ofthe solvent of the non-aqueous electrolytic solution takes place on thesurface of the negative electrode in the charging stage. The reductivedecomposition on the negative electrode takes place after repeatedcharging-discharging procedures even in the case of using EC which isgenerally employed in the non-aqueous electrolytic solution.

[0006] Accordingly, the battery performances such as cycle performanceand electric capacity are at present considered to be not satisfactory.

[0007] When the lithium secondary battery is so overcharged as exceedingan ordinary working voltage, an excessive amount of lithium is releasedfrom the positive electrode, while an excessive amount of lithiumproduces dendrite on the negative electrode. Therefore, both of thepositive electrode and negative electrode become chemically unstable.When both of the positive electrode and negative electrode becomechemically unstable, the carbonate in the non-aqueous electrolyticsolution rapidly decomposes under exothermic reaction. The rapidexothermic reaction causes abnormal heat production of the battery andimpairs safety of the battery. These problems become more serious, asthe energy density of the lithium secondary battery increases. However,at present, the safety in keeping the battery from overcharging and thebattery performances such as cycle performance, electric capacity andstorage endurance are still not satisfactory.

[0008] The present invention has an object to provide a lithiumsecondary battery that is so free from the above-mentioned problems asto be excellent in the safety in overcharging and further in batteryperformances such as cycle performance, electric capacity and storageendurance under the charged conditions, and to provide a novelnon-aqueous electrolytic solution favorably employable for thepreparation of the lithium secondary battery.

DISCLOSURE OF INVENTION

[0009] The present invention relates to a non-aqueous electrolyticsolution comprising an electrolyte dissolved in a non-aqueous solvent,characterized in that the non-aqueous electrolytic solution contains atleast one ketone compound having the formula (I):

[0010] [in the formula, each of R¹, R², R³, R⁴, R⁵, and R⁶ independentlyrepresents a hydrogen atom or an alkyl group having 1-12 carbon atoms;or R¹, R², R³, R⁴, R⁵, and R⁶ are combined together to form acycloalkanone having 4-16 carbon atoms; provided that a total number ofhydrogen atoms for R¹, R² and R³ is 0 or 1] and to a lithium secondarybattery employing the non-aqueous electrolytic solution.

[0011] In more detail, the present invention resides in a non-aqueouselectrolytic solution for lithium secondary batteries comprising anelectrolyte dissolved in a non-aqueous solvent, which further contains aketone compound having the above-mentioned formula (I)[wherein each ofR¹ and R² independently represents a linear or branched alkyl grouphaving 1-12 carbon atoms; and each of R³, R⁴, R⁵ and R⁶ independentlyrepresents a hydrogen atom or a linear or branched alkyl group having1-12 carbon atoms; provided that R¹ and R⁴ can be combined to form acycloalkanone ring having a ring-constituting 4-16 carbon atoms inconjunction with a propanone skeleton to which R¹ and R⁴ are connected,that two or more of an alkyl group of R², an alkyl group of R⁵, abranched chain of an alkyl group of R¹, and a branched chain of an alkylgroup of R⁴ can be combined to form a cycloalkane ring having aring-constituting 4-16 carbon atoms, or that an alkyl group of R¹ and analkyl group of R² and/or an alkyl group of R¹ and an alkyl group of R⁵can be combined to each other to form a cycloalkane ring having aring-constituting 3-16 carbon atoms] and a lithium secondary batteryusing the non-aqueous electrolytic solution.

[0012] As examples of the ketone compounds employed in the invention,the following ketone compounds are mentioned:

[0013] (1) A ketone compound in which each of R¹ and R² independently isa linear or branched alkyl group having 1 to 6 carbon atoms.

[0014] (2) A ketone compound in which R¹ and R⁴ are combined to form acycloalkanone ring having a ring-constituting 4-8 carbon atoms inconjunction with a propanone skeleton to which R¹ and R⁴ are connected(it is preferred that the ketone compound has 2 to 6 substituents on thecycloalkanone ring).

[0015] (3) A ketone compound in which R¹ and R⁴ are combined to form acycloalkanone ring having a ring-constituting 4-8 carbon atoms inconjunction with a propanone skeleton to which R¹ and R⁴ are connected,and further in which two or more of an alkyl group of R², an alkyl groupof R⁵, a branched chain of an alkyl group of R¹, and a branched chain ofan alkyl group of R⁴ are combined to form 1 to 3 cycloalkane ringshaving a ring-constituting 4-8 carbon atoms.

[0016] (4) A ketone compound showing optical isomerism orstereoisomerism.

[0017] Heretofore, there are known the following preventing mechanismsfor overcharge in the lithium secondary battery: a method of performingredox shuttle at a potential of approx. 4.5 V (JP-A-7-302,614); a methodof causing polymerization at a potential of 4.5 V or lower so as toincrease an internal resistance of battery (JP-A-9106,835); and a methodof producing a gas to operate an internal electric switch-off device soas to form an internal shortage, or producing an electroconductivepolymer so as to form an internal shortage, whereby ensuring the safetyof battery under overcharged conditions (JP-A-9-171,840, andJP-A-10-321,258).

[0018] In contrast, the mechanism of preventing overcharge of a lithiumsecondary battery of the invention is considered as follows:

[0019] The aforementioned ketone compound contained in the non-aqueouselectrolytic solution reacts with a lithium metal deposited on thenegative electrode under the overcharged condition to form a passivematerial coat on the negative electrode so that the active lithium metalis inactivated and then further production of active lithium metal issuppressed due to increase of resistance at the negative electrode.Thus, the safety of battery is ensured.

[0020] Further, in the case that the ketone compound contained in thenon-aqueous electrolytic solution has a relatively low oxidationpotential, the ketone compound is electrochemically oxidized on thepositive electrode under the overcharged condition to produce a cation,and the cation moves to the negative electrode on which the cationreacts with the lithium metal deposited on the negative electrode sothat a passive material coat is formed on the negative electrode. As aresult, the active lithium metal is inactivated, and further productionof an active lithium metal is suppressed by increase of resistance onthe negative electrode, whereby the safety of battery is ensured.

[0021] Moreover, since the aforementioned ketone compound contained inthe non-aqueous electrolytic solution has such a high oxidation voltageas+4.5 V to +5.2 V (voltage relative to the oxidation voltage oflithium), the solvent of the electrolytic solution does not decomposeeven when the voltage locally exceeds 4.2 V in the case that thecharge-discharge operation is produced at such a high temperature as 40°C. or higher or at a normal working voltage. Further, the ketonecompound decomposes by reduction on the negative electrode to form astable thin coat on the negative electrode. For these reasons, a lithiumsecondary battery having not only excellent battery safety under theovercharged condition but also excellent battery performances such ascycle performance, electric capacity and storage endurance can beprovided.

[0022] In the formula (I), R¹ and R² independently represents a linearor branched alkyl group having 1-12 carbon atoms; and R³ represents ahydrogen atom or a linear or branched alkyl group having 1-12 carbonatoms. Examples of the alkyl groups include linear alkyl groups such asmethyl, ethyl, propyl, butyl, pentyl, hexyl, and dodecyl and branchedalkyl groups such as isopropyl, isobutyl, sec-butyl, tert-butyl and1,1-dimethylpropyl.

[0023] Each of R⁴, R¹ and R⁶ independently represents a hydrogen atom ora linear or branched alkyl group having 1-12 carbon atoms. Examples ofthe alkyl groups include linear alkyl groups such as methyl, ethyl,propyl, butyl, pentyl, hexyl, and dodecyl and branched alkyl groups suchas isopropyl, isobutyl, sec-butyl, tert-butyl and 1,1-dimethylpropyl.

[0024] Further, R¹ and R⁴ can be combined to each other to form acycloalkanone ring having 4-16 carbon atoms. Examples of thecycloalkanone ring having 4-16 carbon atoms include cyclobutanone,cyclopentanone, cyclohexanone, cycloheptanone, cyclododecanone,cyclooctanone, cyclononanone, and their bicyclo and tricyclo compounds.

[0025] Examples of the ketone compounds of the formula (I) include3-methyl-2-butanone, 2-methyl-3-pentanone, 2,4-dimethyl-3-pentanone,3-methyl-2-pentanone, 4-methyl-3-hexanone, 3,5-dimethyl-4-heptanone,3,3-dimethyl-2-pentanone, pinacoline, 2,2-dimethyl-3-pentanone,2,2,4-trimethyl-3-pentanone, 2,2,4,4-tetramethyl-3-pentanone,3,4-dimethyl-2-pentanone, 3,5-dimethyl-2-hexanone,3,4-dimethyl-2-hexanone, 3-isopropyl-2-heptanone,2,4-dimethylcyclobutanone, 2,2,4,4-tetramethylcyclobutanone,2,5-dimethylcyclopentanone, 2,2,5,5-tetramethylcyclopentanone,(−)-thujone, 2,6-dimethylcyclohexanone,2,2,6,6-tetramethylcyclohexanone, 2,6-di-tert-butylcyclohexanone,2,6-di-sec-butylcyclohexanone, 2-sec-butylcyclohexanone, (−)-menthone,(+)-menthone, (±)-menthone, isomenthone, (−)-camphor, (+)-camphor,(±)-camphor, (+)-nopinone, 2,7-dimethylcycloheptanone,2,2,7,7-tetramethylcycloheptanone, (−)-fenchone, (+)-fenchone,(±)-fenchone, and 2-adamantanone. The ketone compounds can be employedsingly or in optional combinations of two or more compounds.

[0026] R³ of the ketone compound of the formula (I) is either a hydrogenatom or an alkyl group. They are concretely classified in the followingmanner.

[0027] (1) Compounds in which R³ is not a Hydrogen Atom

[0028] (1-1) Examples of the compounds in which all of R⁴, R¹, and R⁶are alkyl groups include cyclic ketone compounds such as theaforementioned 2,2,4,4-tetramethylcyclobutanone,2,2,5,5-tetramethylcyclopentanone, 2,2,6,6-tetramethylcyclohexanone,2,2,7,7-tetramethylcycloheptanone, (−)-fenchone, (+)-fenchone, and(±)-fenchone, and acyclic ketone compounds such as the aforementioned2,2,4,4-tetramethyl-3-pentanone.

[0029] (1-2) Examples of the compounds in which one of R⁴, R⁵ and R⁶ isa hydrogen atom include acyclic ketone compounds such as theaforementioned 2,2,4-trimethyl-3-pentanone and cyclic ketone compoundssuch as 2,2,5-trimethylcyclopetanone and 2,2,6-trimethylcyclohexanone.

[0030] (1-3) Examples of the compounds in which two of R⁴, R⁵ and R⁶ arehydrogen atoms include cyclic ketone compounds such as theaforementioned (−)-camphor, (+)-camphor, and (±)-camphor, and acyclicketone compounds such as the aforementioned 2,2-dimethyl-3-pentanone.

[0031] (1-4) Examples of the compounds in which all of R⁴, R⁵ and R⁶ arehydrogen atoms include acyclic ketone compounds such as theaforementioned 3,3-dimethyl-2-pentanone and pinacoline, and cyclicketone compounds such as methyl-1-methylcyclopropyl ketone and1-acetyladamantane.

[0032] (2) Compounds in which R³ is a Hydrogen Atom

[0033] (2-1). Examples of the compounds in which one of R⁴, R⁵ and R⁶ isa hydrogen atom include cyclic ketone compounds such as theaforementioned 2,4-dimethylcyclobutanone, 2,5-dimethylcyclopentanone,(−)-thujone, 2,6-dimethylcyclohexanone, 2,6-di-tert-butylcyclohexanone,2,6-di-sec-butylcyclohexanone, 2,7-dimethylcycloheptanone and2-adamantanone, dicyclopropyl ketone, and dicyclohexyl ketone, andacyclic ketone compounds such as the aforementioned2,4-dimethyl-3-pentanone and 3,5-dimethyl-4-heptanone.

[0034] (2-2) Examples of the compounds in which two of R⁴, R⁵ and R⁶ arehydrogen atoms include cyclic ketone compounds such as theaforementioned 2-sec-butylcyclohexanone, (−)-menthone, (+)-menthone,(±)-menthone, isomenthone, and (+)-nopinone, and acyclic ketonecompounds such as the aforementioned 2-methyl-3-pentanone and4-methyl-3-hexanone.

[0035] (2-3) Examples of the compounds in which all of R⁴, R⁵ and R⁶ arehydrogen atoms include acyclic ketone compounds such as theaforementioned 3-methyl-2-butanone, 3,4-dimethyl-2-pentanone,3,5-dimethyl-2-hexanone, 3,4-dimethyl-2-hexanone,3-isopropyl-2-heptanone, and 3-methyl-2-pentanone.

[0036] If the amount of a ketone compound of the formula (I) in thenon-aqueous electrolytic solution is excessively large, the batteryperformance may be impaired. If the amount is too small, the expectedbattery performance cannot be obtained. Accordingly, the amount is inthe range of 0.1 to 20 wt. %, preferably 0.2 to 10 wt. %, morepreferably 0.5 to 5 wt. %, from the viewpoint of increase of the cycleperformance.

[0037] Examples of the non-aqueous solvents include cyclic carbonatessuch as ethylene carbonate (EC), propylene carbonate (PC), butylenecarbonate (BC), and vinylene carbonate (VC), lactones such asγ-butyrolactone, linear carbonates such as dimethyl carbonate (DMC),methyl ethyl carbonate (MEC), and diethyl carbonate (DEC), ethers suchas tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane,1,2-dimethoxyethane, 1,2-diethoxyethane, and 1,2-dibutoxyethane,nitriles such as acetonitrile, esters such as methyl propionate, methylpivalate and octyl pivalate, and amides such as dimethylformamide.

[0038] The solvents can be used singly or in combinations of two ormore. There are no specific limitations with respect to the combinationsof the non-aqueous solvents. For instance, there can be mentioned acombination of a cyclic carbonate and a linear carbonate, a combinationof a cyclic carbonate and a lactone, and a combination of three cycliccarbonates and a linear carbonate.

[0039] In order to enhance the overcharge preventing effect, thenon-aqueous electrolytic solution can be mixed with 0.1-5 wt. % of atleast one organic compound selected from biphenyl, 4-methylbiphenyl,4-ethylbiphenyl, o-terphenyl, m-terphenyl, p-terphenyl, andcyclohexylbenzene.

[0040] Examples of the electrolytes include LiPF₆, LiBF₄, LiClO₄,LiN(SO₂CF₃)₂, LiN(SO₂C₂F₅)₂, LiC(SO₂CF₃)₃, LiPF₄(CF₃)₂, LiPF₃(C₂F₅)₃,LiPF₃(CF₃)₃, LIPF₃(iso-C₃F₇)₃, and LiPF₅(iso-C₃F₇). The electrolytes canbe employed singly or in combinations of two ore more. Generally, theelectrolyte can be dissolved in the non-aqueous solvent in such anamount to give an electrolytic solution of 0.1 M to 3 M, preferably 0.5M to 1.5 M.

[0041] The electrolytic solution of the invention can be obtained bymixing the non-aqueous solvents, dissolving the electrolyte in themixture, and dissolving a ketone compound of the formula (I).

[0042] The electrolytic solution of the invention can be favorablyemployed as a component material of a secondary battery, especially alithium secondary battery. There are no specific limitations withrespect to other component materials of the secondary battery, andvarious conventional component materials can be employed.

[0043] For instance, the positive electrode active material can be acomplex metal oxide of lithium and cobalt or nickel. The positiveelectrode active materials can be used singly or in combinations of twoor more. Examples of the complex metal oxides include LiCoO₂, LiNiO₂ andLiCO_(1-x)Ni_(x)O₂ (0.01<x<1). Further, a mixture of LiCoO₂ and LiMn₂O₄,a mixture of LiCoO₂ and LiNiO₂, and a mixture of LiMn₂O₄ and LiNiO₂ canbe used.

[0044] The positive electrode can be prepared by kneading theabove-mentioned positive electrode active material with anelectroconductive agent such as acetylene black or carbon black, abinder such as poly(tetrafluoroethylene) (PTFE), poly(vinylidenefluoride) (PVDF), styrenebutadiene copolymer (SBR),acrylonitrile-butadiene copolymer (NBR) or carboxymethylcellulose (CMC),and a solvent, to give a positive electrode material composition;coating the composition on a collector such as aluminum foil orstainless steel lath plate; drying the coated composition; pressing itfor molding, and heating the molded product at a temperature of approx.50° C. to 250° C., under vacuum, for approx. 2 hours.

[0045] As a negative electrode active material, a lithium metal, alithium alloy, a carbonaceous material having a graphite-typecrystalline structure which can absorb and release lithium [thermallydecomposed carbonaceous material, coke, graphite (e.g., artificialgraphite and natural graphite), fired organic polymer material, andcarbon fiber], or a complex tin oxide. Preferred is a carbonaceousmaterial having a graphite-type crystalline structure in which thelattice distance of lattice surface (002), namely, d₀₀₂, is 0.335-0.340nm. The negative electrode active materials can be used singly or incombination with two or more.

[0046] The negative electrode active material in the powdery form suchas carbonaceous powder is preferably used in combination with a bindersuch as ethylene propylene diene terpolymer (EPDM),poly(tetrafluoroethylene) (PTFE), poly(vinylidene fluoride) (PVDF),styrene-butadiene copolymer (SBR), acrylonitrile-butadiene copolymer(NBR) or carboxymethylcellulbse (CMC), to give a negative electrodematerial composition. There are no limitations with respect to thepreparation of the negative electrode, and the negative electrode can beprepared by a process similar to the aforementioned process forpreparing the positive electrode.

[0047] There are no specific limitations with respect to the structureof the lithium secondary battery of the invention. For instance, thenon-aqueous secondary battery can be a battery of coin type comprising asingle-layered or multi-layered positive electrode, a negativeelectrode, and a separator; a polymer battery; or a cylindrical orprismatic battery comprising a positive electrode roll, a negativeelectrode roll, and a separator roll. The separator can be a knownmicroporous polyolefin film, woven fabric, or non-woven fabric.

EXAMPLE 1

[0048] 1) Preparation of Non-Aqueous Electrolytic Solution

[0049] In a non-aqueous mixture of EC:MEC [30:70, volume ratio] wasdissolved LiPF₆ to give a non-aqueous electrolytic solution of 1 Mconcentration. To the electrolytic solution was added (−)-fenchone in anamount of 1.0 wt. % (based on the amount of the electrolytic solution).

[0050] 2) Preparation of Lithium Secondary Battery and Measurement ofits Battery Performances

[0051] LiCoO₂ (positive electrode active material, 80 wt. %), acetyleneblack (electro-conductive material, 10 wt. %), and poly(vinylidenefluoride) (binder, 10 wt. %) were mixed. To the resulting mixture wasfurther added 1-methyl-2-pyrrolidone (solvent). Thus produced positiveelectrode mixture was coated on aluminum foil, dried, molded underpressure, and heated to give a positive electrode.

[0052] An artificial graphite (negative electrode active material, 90wt. %) and poly(vinylidene fluoride) (binder, 10 wt. %) were mixed. Theresulting mixture was further mixed with 1-methyl-2-pyrrolidone(solvent). Thus produced negative electrode mixture was coated on copperfoil, dried, molded under pressure, and heated to give a negativeelectrode.

[0053] The positive and negative electrodes, a microporous polypropylenefilm separator, and the non-aqueous electrolytic solution were combinedto give a coin-type battery (diameter: 20 mm, thickness: 3.2 mm).

[0054] The coin-type battery was charged at room temperature (20° C.)with a constant electric current (0.8 mA, per electrode area) to reach4.2 V and then the charging was continued under a constant voltage of4.2 V. The charging was performed for 5 hours. Subsequently, the batterywas discharged to give a constant electric current (0.8 mA). Thedischarge was continued to give a terminal voltage of 2.7 V. Thecharge-discharge cycle was repeated.

[0055] The initial discharge capacity was 1.03 times as much as thatmeasured in a battery using a non-aqueous electrolytic solutioncomprising a solvent mixture of EC/MEC (30/70, volume ratio) and IMLiPF₆ but no ketone compound) [see Comparison Example 1].

[0056] After the 50 cycle charge-discharge procedure was complete, thebattery performances were measured. The discharge capacity was 92.4% ofthe initial discharge capacity (100%).

[0057] The coin-type battery was further subjected to another 50 cyclecharge-discharge procedures, and then subjected to a overcharging testby continuing the charging at room temperature (20° C.) at a constantcurrent of 0.8 mA from the fully-charged state. After the overchargingtest, a lithium on the negative electrode was inactivated, and was gray.The low temperature performance was also good.

[0058] The preparation and performances of the coin-type battery are setforth in Table 1.

EXAMPLE 2

[0059] The procedures of Example 1 were repeated except that(−)-fenchone was employed for the electrolytic solution in an amount of0.5 wt. %, to prepare a coin-type battery.

[0060] A battery performance after 50 cycles was measured. The dischargecapacity retention was 85.9%. After the overcharging test, a lithium onthe negative electrode was inactivated, and was gray.

[0061] The preparation and performances of the coin-type battery are setforth in Table 1.

EXAMPLE 3

[0062] The procedures of Example 1 were repeated except that(−)-fenchone was employed for the electrolytic solution in an amount of3.0 wt. %, to prepare a coin-type battery.

[0063] A battery performance after 50 cycles was measured. The dischargecapacity retention was 90.3%. After the overcharging test, a lithium onthe negative electrode was inactivated, and was gray.

[0064] The preparation and performances of the coin-type battery are setforth in Table 1.

EXAMPLE 4

[0065] The procedures of Example 1 were repeated except that(−)-fenchone was employed for the electrolytic solution in an amount of5.0 wt. %, to prepare a coin-type battery.

[0066] A battery performance after 50 cycles was measured. The dischargecapacity retention was 88.1%. After the overcharging test, a lithium onthe negative electrode was inactivated, and was gray.

[0067] The preparation and performances of the coin-type battery are setforth in Table 1.

EXAMPLE 5

[0068] The procedures of Example 1 were repeated except that pinacolinewas employed for the electrolytic solution in an amount of 1.0 wt. %, toprepare a coin-type battery.

[0069] A battery performance after 50 cycles was measured. The dischargecapacity retention was 97.2%. After the overcharging test, a lithium onthe negative electrode was inactivated, and was gray.

[0070] The preparation and performances of the coin-type battery are setforth in Table 1.

EXAMPLE 6

[0071] The procedures of Example 1 were repeated except that2,4-dimethyl-3-pentanone was employed for the electrolytic solution inan amount of 2.0 wt. %, to prepare a coin-type battery.

[0072] A battery performance after 50 cycles was measured. The dischargecapacity retention was 95.0%. After the overcharging test, a lithium onthe negative electrode was inactivated, and was gray.

[0073] The preparation and performances of the coin-type battery are setforth in Table 1.

EXAMPLE 7

[0074] The procedures of Example 1 were repeated except that2,2,4,4-tetramethyl-3-pentanone was employed for the electrolyticsolution in an amount of 1.0 wt. %, to prepare a coin-type battery.

[0075] A battery performance after 50 cycles was measured. The dischargecapacity retention was 85.6%. After the overcharging test, a lithium onthe negative electrode was inactivated, and was gray.

[0076] The preparation and performances of the coin-type battery are setforth in Table 1.

EXAMPLE 8

[0077] The procedures of Example 1 were repeated except that3-isopropyl-2-heptanone was employed for the electrolytic solution in anamount of 2.0 wt. %, to prepare a coin-type battery.

[0078] A battery performance after 50 cycles was measured. The dischargecapacity retention was 96.4%. After the overcharging test, a lithium onthe negative electrode was inactivated, and was gray.

[0079] The preparation and performances of the coin-type battery are setforth in Table 1.

EXAMPLE 9

[0080] The procedures of Example 1 were repeated except that2-adamantanone was employed for the electrolytic solution in an amountof 1.0 wt. %, to prepare a coin-type battery.

[0081] A battery performance after 50 cycles was measured. The dischargecapacity retention was 85.3%. After the overcharging test, a lithium onthe negative electrode was inactivated, and was gray.

[0082] The preparation and performances of the coin-type battery are setforth in Table 1.

EXAMPLE 10

[0083] The procedures of Example 1 were repeated except that(−)-menthone was employed for the electrolytic solution in an amount of1.0 wt. %, to prepare a coin-type battery.

[0084] A battery performance after 50 cycles was measured. The dischargecapacity retention was 88.5%. After the overcharging test, a lithium onthe negative electrode was inactivated, and was gray.

[0085] The preparation and performances of the coin-type battery are setforth in Table 1.

EXAMPLE 11

[0086] The procedures of Example 1 were repeated except that (−)-camphorwas employed for the electrolytic solution in an amount of 1.0 wt. %, toprepare a coin-type battery.

[0087] A battery performance after 50-cycles was measured. The dischargecapacity retention was 93.7%. After the overcharging test, a lithium onthe negative electrode was inactivated, and was gray.

[0088] The preparation and performances of the coin-type battery are setforth in Table 1.

EXAMPLE 12

[0089] The procedures of Example 1 were repeated except that LiPF₆ wasdissolved in a non-aqueous mixture of EC:DEC [30:70, volume ratio] togive a non-aqueous electrolytic solution of 1 M concentration and that(+)-fenchone was employed for the electrolytic solution in an amount of1.0 wt. %, to prepare a coin-type battery.

[0090] A battery performance after 50 cycles was measured. The dischargecapacity retention was 92.6%. After the overcharging test, a lithium onthe negative electrode was inactivated, and was gray.

[0091] The preparation and performances of the coin-type battery are setforth in Table 1.

EXAMPLE 13

[0092] The procedures of Example 1 were repeated except that LiPF₆ wasdissolved in a non-aqueous mixture of EC:DEC [30:70, volume ratio] togive a non-aqueous electrolytic solution of 1 M concentration and that(+)-camphor was employed for the electrolytic solution in an amount of1.0 wt. %, to prepare a coin-type battery.

[0093] A battery performance after 50 cycles was measured. The dischargecapacity retention was 93.6%. After the overcharging test, a lithium onthe negative electrode was inactivated, and was gray.

[0094] The preparation and performances of the coin-type battery are setforth in Table 1.

COMPARISON EXAMPLE 1

[0095] In a non-aqueous mixture of EC:MEC [30:70, volume ratio] wasdissolved LiPF₆ to give a non-aqueous electrolytic solution of 1 Mconcentration. To the electrolytic solution was added no ketonecompound.

[0096] The procedures of Example 1 were repeated except for employingthe above-mentioned electrolytic solution, to prepare a coin-typebattery. A battery performance was then measured.

[0097] The discharge capacity retention after the 50 cycle test was82.6%. After the overcharging test, a lithium on the negative electrodewas not inactivated, and dendrite deposited on the electrode. Thepreparation and performances of the coin-type battery are set forth inTable 1.

COMPARISON EXAMPLE 2

[0098] The procedures of Example 1 were repeated except that acetone wasemployed for the electrolytic solution in an amount of 1.0 wt. %, toprepare a coin-type battery.

[0099] A battery performance was measured. The discharge capacityretention after the 50 cycle test was 1.1%.

[0100] The preparation and performances of the coin-type battery are setforth in Table 1.

COMPARISON EXAMPLE 3

[0101] The procedures of Example 1 were repeated except thatcyclohexanone was employed for the electrolytic solution in an amount of1.0 wt. %, to prepare a coin-type battery. A battery performance wasmeasured. The discharge capacity retention after the 50 cycle test was0.4%. The preparation and performances of the coin-type battery are setforth in Table 1.

EXAMPLE 14

[0102] The procedures of Example 1 were repeated except that thenon-aqueous electrolytic solution was replaced with 1 MLiPF₆-EC/PC/MEC/DMC (30/5/50/15, volume ratio] and that LiCoO₂ (positiveelectrode active material) was replaced with LiNi_(0.8)CO_(0.2)O₂, toprepare a coin-type battery.

[0103] A battery performance after 50 cycles was measured. The dischargecapacity retention was 91.1%. After the overcharging test, a lithium onthe negative electrode was inactivated, and was gray.

[0104] The preparation and performances of the coin-type battery are setforth in Table 1.

EXAMPLE 15

[0105] The procedures of Example 1 were repeated except that thenon-aqueous electrolytic solution was replaced with 1 MLiBF₄-EC/PC/DEC/DMC (30/5/30/35, volume ratio] and that LiCoO₂ (positiveelectrode active material) was replaced with LiMn₂O₄, to prepare acoin-type battery.

[0106] A battery performance after 50 cycles was measured. The dischargecapacity retention was 92.5%. After the overcharging test, a lithium onthe negative electrode was inactivated, and was gray.

[0107] The preparation and performances of the coin-type battery are setforth in Table 1. TABLE 1 Initial discharge Discharge Compound capacitycapacity (wt. %) (R.V.) retention (%) Ex. 1 (−)-fenchone (1.0) 1.03 92.4Ex. 2 (−)-fenchone (0.5) 1.01 85.9 Ex. 3 (−)-fenchone (3.0) 1.02 90.3Ex. 4 (−)-fenchone (5.0) 0.98 88.1 Ex. 5 pinacoline (1.0) 1.00 97.2 Ex.6 2,4-dimethyl- (2.0) 0.97 95.0 pentanone Ex. 7 2,2,4,4-tetramethyl-(1.0) 1.01 85.6 3-pentanone Ex. 8 3-isopropyl-2- (2.0) 0.93 96.4heptanone Ex. 9 2-adamantanone (1.0) 0.96 85.3 Ex. 10 (−)-menthone (1.0)1.01 88.5 Ex. 11 (−)-camphor (1.0) 1.02 93.7 Ex. 12 (+)-fenchone (1.0)1.02 92.6 Ex. 13 (+)-camphor (1.0) 1.01 93.6 Com. 1 none 1.00 82.6 Com.2 acetone (1.0) 0.82 1.1 Com. 3 cyclohexanone (1.0) 0.83 0.4 Ex. 14(−)-fenchone (1.0) 1.06 91.1 Ex. 15 (−)-fenchone (1.0) 0.99 92.5

[0108] Remarks:

[0109] Positive electrode: LiCoO₂ for Examples 1-13 and ComparisonExamples 1-3; LiNi_(0.8)CO_(0.2)O₂ for Example 14: and LiMn₂O₄ forExample 15.

[0110] Negative electrode: artificial graphite for all

EXAMPLES AND COMPARISON EXAMPLES.

[0111] Electrolytic solution: 1M LiPF₆-EC/MEC=30/70 (volume ratio) forExamples 1-11 and Comparison Examples 1-3; 1M LiPF₆-EC/DEC=30/70 (volumeratio) for Examples 12 and 13; 1M LiPF₆-EC/PC/MEC/DMC=30/5/50/15 (volumeratio) for Example 14; and 1M LiBF₄ EC/PC/DEC/DMC=30/5/30/35 (volumeratio) for Example 15.

[0112] R.V.: Relative Value.

[0113] The chemical formulas of (−)-fenchone, pinacoline,2,4-dimethyl-3-pentanone, 2,2,4,4-tetramethyl-3-pentanone,3-isopropyl-2-heptanone, 2-adamantanone, (−)-menthone, (−)-camphor, and(+)-camphor are described below:

[0114] The present invention is not limited to the above-mentionedexamples, and various combinations inferred from the gist of theinvention are utilizable. Particularly, no limitations with respect tothe combinations of the solvents are given by the those of Examples.Further, the present invention is applicable to batteries ofcylindrical-type and prismatic-type, and polymer battery. [IndustrialUtilization of Invention]

[0115] The present invention provides a lithium secondary battery thatis excellent in battery safety and battery performances such as cycleperformance, electric capacity, and storage endurance.

1. A non-aqueous electrolytic solution for lithium secondary batteriescomprising an electrolyte dissolved in a non-aqueous solvent, whichfurther contains a ketone compound having the formula (I):

wherein each of R¹ and R² independently represents a linear or branchedalkyl group having 1-12 carbon atoms; and each of R³, R⁴, R⁵ and R⁶independently represents a hydrogen atom or a linear or branched alkylgroup having 1-12 carbon atoms; provided that R¹ and R⁴ can be combinedto form a cycloalkanone ring having a ring-constituting 4-16 carbonatoms in conjunction with a propanone skeleton to which R¹ and R⁴ areconnected, that two or more of an alkyl group of R², an alkyl group ofR⁵, a branched chain of an alkyl group of R¹, and a branched chain of analkyl group of R⁴ can be combined to form a cycloalkane ring having aring-constituting 4-16 carbon atoms, or that an alkyl group of R¹ and analkyl group of R² and/or an alkyl group of R⁴ and an alkyl group of R⁵can be combined to each other to form a cycloalkane ring having aring-constituting 3-16 carbon atoms:
 2. The non-aqueous electrolyticsolution of claim 1, wherein the ketone compound has the formula (I) inwhich each of R¹ and R² independently is a linear or branched alkylgroup having 1 to 6 carbon atoms.
 3. The non-aqueous electrolyticsolution of claim 1, wherein the ketone compound has the formula (I) inwhich R¹ and R⁴ are combined to form a cycloalkanone ring having aring-constituting 4-8 carbon atoms in conjunction with a propanoneskeleton to which R¹ and R⁴ are connected.
 4. The non-aqueouselectrolytic solution of claim 3, wherein the ketone compound has 2 to 6substituents on the cycloalkanone ring.
 5. The non-aqueous electrolyticsolution of claim 1, wherein the ketone compound has the formula (I) inwhich R¹ and R⁴ are combined to form a cycloalkanone ring having aring-constituting 4-8 carbon atoms in conjunction with a propanoneskeleton to which R¹ and R⁴ are connected, and further in which two ormore of an alkyl group of R², an alkyl group of R⁵, a branched chain ofan alkyl group of R¹, and a branched chain of an alkyl group of R⁴ arecombined to form 1 to 3 cycloalkane rings having a ring-constituting 4-8carbon atoms.
 6. The non-aqueous electrolytic solution of any one ofclaims 1 to 5, wherein the ketone compound is an optical isomer or astereoisomer.
 7. The non-aqueous electrolytic solution of claim 1,wherein the ketone compound is selected from the group consisting offenchone, pinacoline, 2,4-dimethyl-3-pentanone,2,2,4,4-tetramethyl-3-pentanone, 3-isopropyl-2-heptanone,2-adamantanone, menthone, and camphor.
 8. The non-aqueous electrolyticsolution of any one of claims 1 to 7, wherein the ketone compound iscontained in an amount of 0.2 to 10 wt. %.
 9. The non-aqueouselectrolytic solution of claim 8, wherein the ketone compound iscontained in an amount of 0.5 to 5 wt. %.
 10. The non-aqueouselectrolytic solution of any one of claims 1 to 9, wherein thenon-aqueous solvent is a mixture solvent comprising at least a cycliccarbonate and a linear carbonate.
 11. The non-aqueous electrolyticsolution of any one of claims 1 to 10, wherein the electrolyte isselected from the group consisting of LiPF₆, LiBF₄, LiClO₄, LiN(SO₂CF₃)₂, LiN(SO₂C₂F₅)₂, LiC(SO₂CF₃)₃, LiPF₄(CF₃)₂, LiPF₃(C₂F₅)₃,LiPF₃(CF₃)₃, LiPF₃(iso-C₃F₇)₃, and LiPF₅(iso-C₃F₇).
 12. A lithiumsecondary battery comprising a positive electrode, a negative electrode,and a non-aqueous electrolytic solution comprising an electrolytedissolved in a non-aqueous solvent, wherein the non-aqueous electrolyticsolution for lithium secondary batteries comprises an electrolytedissolved in a non-aqueous solvent, which further contains a ketonecompound having the formula (I):

wherein each of R¹ and R² independently represents a linear or branchedalkyl group having 1-12 carbon atoms; and each of R³, R⁴, R⁵ and R⁶independently represents a hydrogen atom or a linear or branched alkylgroup having 1-12 carbon atoms; provided that R¹ and R⁴ can be combinedto form a cycloalkanone ring having a ring-constituting 4-16 carbonatoms in conjunction with a propanone skeleton to which R¹ and R⁴ areconnected, that two or more of an alkyl group of R², an alkyl group ofR¹, a branched chain of an alkyl group of R¹, and a branched chain of analkyl group of R⁴ can be combined to form a cycloalkane ring having aring-constituting 4-16 carbon atoms, or that an alkyl group of R¹ and analkyl group of R² and/or an alkyl group of R⁴ and an alkyl group of R⁵can be combined to each other to form a cycloalkane ring having aring-constituting 3-16 carbon atoms.
 13. The lithium secondary batteryof claim 12, wherein the ketone compound has the formula (I) in whicheach of R¹ and R² independently is a linear or branched alkyl grouphaving 1 to 6 carbon atoms.
 14. The lithium secondary battery of claim12, wherein the ketone compound has the formula (I) in which R¹ and R⁴are combined to form a cycloalkanone ring having a ring-constituting 4-8carbon atoms in conjunction with a propanone skeleton to which R¹ and R⁴are connected.
 15. The lithium secondary battery of claim 14, whereinthe ketone compound has 2 to 6 substituents on the cycloalkanone ring.16. The lithium secondary battery of claim 12, wherein the ketonecompound has the formula (I) in which R¹ and R⁴ are combined to form acycloalkanone ring having a ring-constituting 4-8 carbon atoms inconjunction with a propanone skeleton to which R¹ and R⁴ are connected,and further in which two or more of an alkyl group of R², an alkyl groupof R⁵, a branched chain of an alkyl group of R¹, and a branched chain ofan alkyl group of R⁴ are combined to form 1 to 3 cycloalkane ringshaving a ring-constituting 4-8 carbon atoms.
 17. The lithium secondarybattery of claim 12, wherein the ketone compound is selected from thegroup consisting of fenchone, pinacoline, 2,4-dimethyl-3-pentanone,2,2,4,4-tetramethyl-3-pentanone, 3-isopropyl-2-heptanone,2-adamantanone, menthone, and camphor.
 18. The lithium secondary batteryof any one of claims 12 to 17, wherein the ketone compound is containedin an amount of 0.2 to 10 wt. %.
 19. The lithium secondary battery ofclaim 18, wherein the ketone compound is contained in an amount of 0.5to 5 wt. %.
 20. The lithium secondary battery of any one of claims 12 to19, wherein the negative electrode comprises artificial graphite as anegative electrode active material.